Attach subdivision surfaces

User Guide > Modeling > Subdivision Surfaces > Editing subdivision surfaces >

The edges of the surfaces will attach only if each surface has the same number of polygonal edges at the base mesh level (level 0). If the surface does not have the same number of edges, the objects are grouped, but the vertices and edges of the surfaces do not merge.

Attaching will destroy any deformer weighting on the selected surfaces.

To prepare subdivision surfaces to be attached

Make sure the two surfaces you want to attach each have the same number of polygonal edges at the base mesh (level 0). If they do not have the same number of edges, the surfaces will be grouped, but will not merge.
Use Split Polygon or Delete Edge on the polygon proxy to create or remove edges.

Switch to Standard mode if you are not already in it ( Subdiv Surfaces > Standard Mode).
If you are merging mirrored surfaces, delete the history on the mirror copy.
Subdiv Surfaces > Attach requires surfaces with consistently oriented normals.
If one of the surfaces you wish to attach has its normals facing the wrong direction, Edit > Duplicate it, Subdiv Surfaces > Mirror it in the opposite direction, and then Modify > Freeze Transformations on the mirrored object.

To attach subdivision surfaces

Select the surfaces you want to attach.
Select Subdiv Surfaces > Attach.

Notes

The edges of the surfaces will attach only if each surface has the same number of polygonal edges at the base mesh level (level 0).
If the surface does not have the same number of edges, the objects are grouped, but the vertices and edges of the surfaces do not merge.
Attaching will destroy any deformer weighting on the selected surfaces.